US9323170B2 - Image forming apparatus with a controller to set transfer bias - Google Patents

Image forming apparatus with a controller to set transfer bias Download PDF

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Publication number
US9323170B2
US9323170B2 US13/483,536 US201213483536A US9323170B2 US 9323170 B2 US9323170 B2 US 9323170B2 US 201213483536 A US201213483536 A US 201213483536A US 9323170 B2 US9323170 B2 US 9323170B2
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Prior art keywords
component
image
toner
superimposed
transfer
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US13/483,536
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US20130004190A1 (en
Inventor
Kenji Sengoku
Hiromi Ogiyama
Hiroyoshi Haga
Tomokazu Takeuchi
Yasunobu Shimizu
Junpei FUJITA
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Ricoh Co Ltd
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Ricoh Co Ltd
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Assigned to RICOH COMPANY, LTD. reassignment RICOH COMPANY, LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: Takeuchi, Tomokazu, Fujita, Junpei, HAGA, HIROYOSHI, SENGOKU, KENJI, SHIMIZU, YASUNOBU, OGIYAMA, HIROMI
Publication of US20130004190A1 publication Critical patent/US20130004190A1/en
Priority to US15/076,031 priority Critical patent/US9864307B2/en
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    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G15/00—Apparatus for electrographic processes using a charge pattern
    • G03G15/14—Apparatus for electrographic processes using a charge pattern for transferring a pattern to a second base
    • G03G15/16—Apparatus for electrographic processes using a charge pattern for transferring a pattern to a second base of a toner pattern, e.g. a powder pattern, e.g. magnetic transfer
    • G03G15/1665—Apparatus for electrographic processes using a charge pattern for transferring a pattern to a second base of a toner pattern, e.g. a powder pattern, e.g. magnetic transfer by introducing the second base in the nip formed by the recording member and at least one transfer member, e.g. in combination with bias or heat
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G15/00—Apparatus for electrographic processes using a charge pattern
    • G03G15/02—Apparatus for electrographic processes using a charge pattern for laying down a uniform charge, e.g. for sensitising; Corona discharge devices
    • G03G15/0266—Arrangements for controlling the amount of charge
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G15/00—Apparatus for electrographic processes using a charge pattern
    • G03G15/01—Apparatus for electrographic processes using a charge pattern for producing multicoloured copies
    • G03G15/0142—Structure of complete machines
    • G03G15/0178—Structure of complete machines using more than one reusable electrographic recording member, e.g. one for every monocolour image
    • G03G15/0189—Structure of complete machines using more than one reusable electrographic recording member, e.g. one for every monocolour image primary transfer to an intermediate transfer belt
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G2215/00—Apparatus for electrophotographic processes
    • G03G2215/01—Apparatus for electrophotographic processes for producing multicoloured copies
    • G03G2215/0103—Plural electrographic recording members
    • G03G2215/0119—Linear arrangement adjacent plural transfer points
    • G03G2215/0122—Linear arrangement adjacent plural transfer points primary transfer to an intermediate transfer belt
    • G03G2215/0125—Linear arrangement adjacent plural transfer points primary transfer to an intermediate transfer belt the linear arrangement being horizontal or slanted
    • G03G2215/0129—Linear arrangement adjacent plural transfer points primary transfer to an intermediate transfer belt the linear arrangement being horizontal or slanted horizontal medium transport path at the secondary transfer

Definitions

  • Exemplary aspects of the present invention generally relate to an electrophotographic image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
  • a charger uniformly charges a surface of an image bearing member (which may, for example, be a photoconductive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to
  • a transfer method known as a direct current (DC) transfer method in which a direct current bias is applied to a transfer device, is widely employed to transfer a toner image onto a recording medium.
  • DC direct current
  • AC transfer method in which a superimposed bias (also known as an AC bias) is applied to the transfer device.
  • the superimposed bias is composed of an alternating current (AC) voltage superimposed on a DC voltage.
  • AC transfer method is more advantageous than the DC transfer method for a recording medium having a coarse surface. It is known that the AC transfer method can enhance transferability and prevent a disturbance of toner image such as dropouts.
  • toner may not be transferred well if the same transfer bias used in a monochrome mode for forming a monochrome image is applied in a color mode for forming a multicolor or full-color image.
  • a transfer voltage for forming a color image is configured greater than a transfer voltage for forming a monochrome image.
  • the transferability does not increase proportional to the transfer voltage in the AC transfer method. More specifically, simply increasing the transfer voltage does not transfer toner well onto a recording medium for a color image that contains a large amount of toner.
  • a novel image forming apparatus including an image bearing member, a transfer device, a transfer bias power source, and a controller.
  • the image bearing member bears a toner image on a surface thereof.
  • the transfer device disposed opposite the image bearing member transfers the toner image from the image bearing member onto a recording medium.
  • the transfer bias power source applies, between the image bearing member and the transfer device, a superimposed transfer bias in which a direct current (DC) component and an alternative current (AC) component are superimposed to transfer the toner image borne on the image bearing member to the recording medium.
  • the controller changes the DC component and the AC component of the superimposed bias that the transfer bias power source applies.
  • the controller changes the DC component and the AC component of the superimposed transfer bias in a color mode from that in a monochrome mode, to secure a return electric field in the superimposed transfer bias by which the toner is returned from the recording medium to the image bearing member.
  • a color mode an image is formed with a plurality of toners.
  • the monochrome mode an image is formed with a toner of a single color.
  • a method for forming an image includes forming a toner image on a surface of an image bearing member; transferring the toner image from the image bearing member onto a recording medium; applying, between the image bearing member and a transfer device, a superimposed transfer bias in which a direct current (DC) component and an alternative current (AC) component are superimposed to transfer the toner image borne on the image bearing member to the recording medium; and changing levels of the DC component and the AC component of the superimposed bias applied by the applying.
  • DC direct current
  • AC alternative current
  • the changing step includes changing the levels of the DC component and the AC component of the superimposed bias in a color mode from that in a monochrome mode, to secure a return electric field in the superimposed transfer bias by which the toner is returned from the recording medium to the image bearing member.
  • a color mode an image is formed with a plurality of toners
  • a monochrome mode an image is formed with a toner of a single color.
  • FIG. 1 is a cross-sectional diagram schematically illustrating an example of an image forming apparatus according to an illustrative embodiment of the present invention
  • FIG. 2 is a cross-sectional diagram schematically illustrating an image forming unit as a representative example of image forming units employed in the image forming apparatus of FIG. 1 according to an illustrative embodiment of the present invention
  • FIG. 3 is a waveform chart showing an example of a waveform of a superimposed bias serving as a secondary transfer bias
  • FIG. 4 is a waveform chart showing an example of a waveform of a bias that transfers toner poorly for a color image
  • FIG. 5 is a waveform chart showing an example of a waveform of a superimposed bias in a color mode according to an illustrative embodiment of the present invention
  • FIG. 6 is a cross-sectional diagram schematically illustrating a color printer of a direct transfer method as an example of an image forming apparatus according to an illustrative embodiment of the invention
  • FIG. 7 is a cross-sectional diagram schematically illustrating a color image forming apparatus employing a single drum-type photosensitive member according to an illustrative embodiment of the present invention.
  • FIG. 8 is a schematic diagram illustrating a variation of a transfer portion of the image forming apparatus.
  • first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that such elements, components, regions, layers and/or sections are not limited thereby because such terms are relative, that is, used only to distinguish one element, component, region, layer or section from another region, layer or section.
  • a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of this disclosure.
  • paper is the medium from which is made a sheet on which an image is to be formed. It should be noted, however, that other printable media are available in sheet form, and accordingly their use here is included. Thus, solely for simplicity, although this Detailed Description section refers to paper, sheets thereof, paper feeder, etc., it should be understood that the sheets, etc., are not limited only to paper, but include other printable media as well.
  • FIG. 1 a description is provided of an image forming apparatus according to an illustrative embodiment of the present invention.
  • FIG. 1 is a schematic diagram illustrating a color printer as an example of the image forming apparatus employing an intermediate transfer method in which a toner image is indirectly transferred onto a recording medium via an intermediate transfer member according to an illustrative embodiment of the present invention.
  • the image forming apparatus includes four image forming units 1 Y, 1 M, 1 C, and 1 K (which may be collectively referred to as image forming units 1 ), an optical writing unit 80 , a transfer unit 50 including an intermediate transfer belt 51 , a fixing device 90 , and so forth.
  • the image forming units 1 Y, 1 M, 1 C, and 1 K are arranged in tandem in the direction of movement of the intermediate transfer belt 51 , thereby constituting a tandem imaging station.
  • suffixes Y, M, C, and K denote the colors yellow, magenta, cyan, and black, respectively. To simplify the description, the suffixes Y, M, C, and K indicating colors are omitted herein unless otherwise specified.
  • FIG. 2 is a schematic diagram illustrating the image forming unit 1 .
  • the image forming unit 1 includes a drum-shaped photosensitive member 11 , a charging device 21 , a developing device 31 , a primary transfer roller 55 , a cleaning device 41 , and so forth.
  • the charging device 21 charges the surface of the photosensitive drum 11 by using a charging roller 21 a .
  • the developing device 31 develops a latent image formed on the photosensitive drum 11 with a respective color of toner to form a visible image known as a toner image.
  • the primary transfer roller 55 serving as a primary transfer member transfers the toner image from the photosensitive drum 11 to the intermediate transfer belt 51 .
  • the cleaning device 41 cleans the surface of the photosensitive drum 11 after primary transfer.
  • the image forming units 1 Y, 1 M, 1 C, and 1 K are detachably attachable relative to the image forming apparatus main body.
  • the photosensitive drum 11 is constituted of a drum-shaped base on which an organic photosensitive layer is disposed.
  • the external diameter of the photosensitive drum is approximately 60 mm.
  • the photosensitive drum 11 is rotated in a clockwise direction indicated by an arrow R 1 by a driving device, not illustrated.
  • the charging roller 21 a of the charging device 21 is supplied with a charging bias.
  • the charging roller 21 a contacts or is disposed close to the photosensitive drum 11 to generate an electrical discharge therebetween, thereby charging uniformly the surface of the photosensitive drum 11 .
  • the photosensitive drum 11 is uniformly charged with a negative polarity which is the same polarity of normal charge on toner.
  • a charging bias an alternating current (AC) voltage superimposed on a direct current (DC) voltage is employed.
  • the photoconductive drum 11 is charged by the charging roller 21 a contacting or disposed near the photoconductive drum 11 .
  • a charger such as a corona charger may be employed.
  • the developing device 31 includes a developing sleeve 31 a , and paddles 31 b and 31 c inside a developer container 31 d .
  • a two-component developing agent consisting of toner particles and carriers is stored in the developer container 31 d .
  • the developing sleeve 31 a serves as a developer bearing member and faces the photoconductive drum 11 via an opening of the developer container 31 d .
  • the paddles 31 b and 31 c mix the developing agent and deliver the developing agent to the developing sleeve 31 a .
  • the two-component developing agent is used.
  • a single-component developing agent may be used.
  • the cleaning device 41 includes a cleaning blade 41 a and a cleaning brush 41 b to clean the surface of the photosensitive drum 11 .
  • the cleaning blade 41 a of the cleaning device 41 contacts the surface of the photosensitive drum 11 at a certain angle such that the leading edge of the cleaning blade 41 a faces counter to the direction of rotation of the photosensitive drum 11 .
  • the cleaning brush 41 b rotates in the direction opposite to the direction of rotation of the photosensitive drum 11 while contacting the photosensitive drum 11 .
  • the optical writing unit 80 for writing a latent image on each of photosensitive drums 11 Y, 11 M, 11 C, and 11 K (which may be referred to collectively as photosensitive drums 11 ) is disposed above the image forming units 1 Y, 1 M, 1 C, and 1 K. Based on image information received from an external device such as a personal computer (PC), the optical writing unit 80 illuminates the photosensitive drum 11 with a light beam projected from a laser diode of the optical writing unit 80 . Accordingly, the electrostatic latent images of yellow, magenta, cyan, and black are formed on the photosensitive drums 11 Y, 11 M, 11 C, and 11 K, respectively.
  • PC personal computer
  • the potential of the portion of the uniformly-charged surface of the photosensitive drums 11 illuminated with the light beam is attenuated.
  • the potential of the illuminated portion of the photosensitive drum 11 with the light beam is less than the potential of the other area, that is, a background portion (no-image portion), thereby forming an electrostatic latent image on the photosensitive drum 11 .
  • the optical writing unit 80 includes a polygon mirror, a plurality of optical lenses, and mirrors.
  • the light beam projected from the laser diode serving as a light source is deflected in a main scanning direction by the polygon mirror rotated by a polygon motor.
  • the deflected light then, strikes the optical lenses and mirrors, thereby scanning the photosensitive drums 11 .
  • the optical writing unit 80 may employ a light source using an LED array including a plurality of LEDs that projects light.
  • the transfer unit 50 is disposed below the image forming units 1 Y, 1 M, 1 C, and 1 K.
  • the transfer unit 50 includes the intermediate transfer belt 51 serving as an image bearing member formed into an endless loop and entrained about a plurality of rollers, thereby rotating endlessly in the counterclockwise direction indicated by arrow A.
  • the transfer unit 50 also includes a driving roller 52 , a secondary transfer roller 53 , a cleaning backup roller 54 , four primary transfer rollers 55 Y, 55 M, 55 C, and 55 K (which may be referred to collectively as primary transfer rollers 55 ), a nip forming roller 56 , a belt cleaning device 57 , an electric potential detector 58 , and so forth.
  • the primary transfer rollers 55 Y, 55 M, 55 C, and 55 K is disposed opposite the photosensitive drums 11 Y, 1 M, 11 C, and 11 K, respectively, via the intermediate transfer belt 51 .
  • the intermediate transfer belt 51 is entrained around and stretched taut between the driving roller 52 , the secondary transfer roller 53 , the cleaning backup roller 54 , and the primary transfer rollers 55 , all disposed inside the loop formed by the intermediate transfer belt 51 .
  • the driving roller 52 is rotated by a driving device (not illustrated), enabling the intermediate transfer belt 51 to move in the direction of arrow A.
  • the intermediate transfer belt 51 is made of resin such as polyimide resin in which carbon is dispersed and has a thickness in a range of from approximately 20 ⁇ m to 200 ⁇ m, preferably, approximately 60 ⁇ m.
  • the surface resistivity thereof is in a range of from approximately 9.0 to 13.0 [Log ⁇ / ⁇ ], preferably, approximately 10.0 to 12.0 [Log ⁇ / ⁇ ].
  • the surface resistivity is measured by using an HRS probe with an applied voltage of 500V.
  • the surface resistivity is calculated after 10 seconds elapsed.
  • the volume resistivity thereof is in a range of from approximately 6.0 to 13.0 [Log ⁇ cm], preferably, approximately 7.5 to 12.5 [Log ⁇ cm], and more preferably, approximately 9 [Log ⁇ cm].
  • the volume resistivity is measured by using the HRS probe with an applied voltage of 100V.
  • the volume resistivity is calculated after 10 seconds elapsed.
  • the intermediate transfer belt 51 is interposed between the photosensitive drums 11 Y, 11 M, 11 C, and 11 K, and the primary transfer rollers 55 . Accordingly, primary transfer nips are formed between the front surface (image bearing surface) of the intermediate transfer belt 51 and the photosensitive drums 11 Y, 11 M, 11 C, and 11 K contacting the intermediate transfer belt 51 .
  • the primary transfer rollers 55 are applied with a primary bias by a transfer bias power source, thereby generating a transfer electric field between the toner images on the photosensitive drums 11 and the primary transfer rollers 55 . Accordingly, the toner images are transferred primarily from the photosensitive drums 11 onto the intermediate transfer belt 51 by the transfer electric field and a nip pressure at the primary transfer nip. More specifically, the toner images of yellow, magenta, cyan, and black are transferred onto the intermediate transfer belt 51 so that they are superimposed atop the other, thereby forming a composite toner image on the intermediate transfer belt 51 .
  • a support plate supporting the primary transfer rollers 55 Y, 55 M, and 55 C of the transfer unit 50 is moved to separate the primary transfer rollers 55 Y, 55 M, and 55 C from the photosensitive drums 11 Y, 11 M, and 11 C. Accordingly, the front surface of the intermediate transfer belt 51 , that is, the image bearing surface, is separated from the photosensitive drums 11 Y, 11 M, and 11 C so that the intermediate transfer belt 51 contacts only the photosensitive drum 11 K. In this state, only the image forming unit 1 K is activated to form a toner image of black on the photosensitive drum 11 K.
  • Each of the primary transfer rollers 55 is constituted of an elastic roller including a metal cored bar on which a conductive sponge layer is provided.
  • the external diameter of the primary transfer roller 55 is approximately 16 mm, and the diameter of the metal cored bar is approximately 10 mm.
  • a primary transfer bias is applied to the primary transfer rollers 55 with constant current control.
  • a roller-type transfer device here, the primary transfer roller 55
  • a transfer charger or a brush-type transfer device may be employed as a primary transfer device.
  • the nip forming roller 56 of the transfer unit 50 is disposed outside the loop formed by the intermediate transfer belt 51 , opposite the secondary transfer roller 53 which is disposed inside the loop.
  • the intermediate transfer belt 51 is interposed between the secondary transfer roller 53 and the nip forming roller 56 . Accordingly, a secondary transfer nip is formed between the peripheral surface or the image bearing surface of the intermediate transfer belt 51 and the nip forming roller 56 contacting the surface of the intermediate transfer belt 51 .
  • the nip forming roller 56 is grounded.
  • a secondary transfer bias is applied to the secondary transfer roller 53 by a secondary transfer bias power source 200 .
  • a sheet cassette 100 storing a stack of recording media sheets P is disposed below the transfer unit 500 .
  • the sheet cassette 100 is equipped with a sheet feed roller 101 to contact a top sheet of the stack of recording media sheets P. As the sheet feed roller 101 is rotated at a predetermined speed, the sheet feed roller 101 picks up the top sheet and sends it to a sheet passage.
  • a pair of registration rollers 102 is disposed.
  • the pair of the registration rollers 102 stops rotating temporarily as soon as the recording medium P delivered from the sheet cassette 100 is interposed therebetween.
  • the pair of registration rollers 102 starts to rotate again to feed the recording medium P to the secondary transfer nip in appropriate timing such that the recording medium P is aligned with a composite or monochrome toner image formed on the intermediate transfer belt 51 in the secondary transfer nip.
  • the recording medium P tightly contacts the composite or monochrome toner image on the intermediate transfer belt 51 , and the composite or monochrome toner image is transferred secondarily onto the recording medium P by the secondary transfer electric field and the nip pressure applied thereto.
  • the recording medium P, on which the composite or monochrome toner image is transferred passes through the secondary transfer nip and separates from the nip forming roller 56 and the intermediate transfer belt 51 due to the elasticity of the recording medium, also known as self stripping.
  • the secondary transfer roller 53 is constituted of a metal cored bar made of metal such as stainless steel and aluminum on which a resistance layer is laminated.
  • Specific preferred materials suitable for the resistance layer include, but are not limited to, polycarbonate, fluorine-based rubber, silicon rubber, and the like in which conductive particles such as carbon and metal complex are dispersed, or rubbers such as nitrile rubber (NBR) and Ethylene Propylene Diene Monomer (EPDM), rubber of NBR/ECO copolymer, and semiconductive rubber such as polyurethane.
  • NBR nitrile rubber
  • EPDM Ethylene Propylene Diene Monomer
  • rubber of NBR/ECO copolymer Rubber of NBR/ECO copolymer
  • semiconductive rubber such as polyurethane.
  • the volume resistance thereof is in a range of from 6.0 to 8.0 [Log ⁇ ], preferably in a range of from 7.0 to 8.0 [Log ⁇ ].
  • the resistance layer may be a foam-type having the hardness in a range of from 20 degrees and 50 degrees or a rubber-type having the hardness in a range of from 30 degrees and 60 degrees.
  • the sponge-type layer is preferred because it reliably contacts the nip forming roller 56 via the intermediate transfer belt 51 even with a low contact pressure.
  • image defects such as toner dropouts can be prevented.
  • Toner dropouts are a partial toner transfer failure in character images or thin-line images.
  • the nip forming roller 56 (a counter roller) is constituted of a metal cored bar made of metal such as stainless steel and aluminum, and a resistance layer and a surface layer made of conductive rubber or the like disposed on the metal cored bar.
  • the external diameter of the nip forming roller 56 is approximately 20 mm, and the diameter of the metal cored bar is approximately 16 mm.
  • the resistant layer is made of rubber of NBR/ECO copolymer having the hardness in the range of from 40 to 60 degrees according to JIS-A.
  • the surface layer is made of fluorinated urethane elastomer.
  • the thickness thereof is preferably in the range of from 8 to 24 ⁇ m. This is because the surface layer of the roller is generally formed during coating process, and if the thickness of the surface layer is less than or equal to 8 ⁇ m, the effect of uneven resistance due to uneven coating is significant. As a result, leak may occur at a place with low resistance. Furthermore, the surface of the roller may wrinkle, causing cracks in the surface layer.
  • the thickness of the surface layer is 24 ⁇ m or more, the resistance becomes high.
  • the voltage may rise and exceed an allowable range of voltage change of the constant current power source when the constant current is supplied to the metal cored bar of the secondary transfer roller 53 .
  • the current may drop below the target value.
  • the allowable range of voltage change is high enough, the voltage of a high-voltage path from the constant current power source to the metal cored bar of the secondary transfer roller and/or the metal cored bar of the secondary transfer roller may become high, causing the leak easily.
  • the thickness of the surface layer of the nip forming roller 56 is 24 ⁇ m or more, the hardness becomes high, thereby hindering the nip forming roller 56 from closely contacting the recording medium P and the intermediate transfer belt 51 .
  • the surface resistance of the nip forming roller 56 is equal to or greater than 10 6.5 ⁇ , and the volume resistance thereof is in a range of from 6.0 to 12.0 Log ⁇ .
  • the volume resistance of the nip forming roller 56 when using a metal roller such as SUS is 4.0 Log ⁇ . The volume resistance is measured using the rotation measurement method as described above.
  • the electric potential detector 58 is disposed outside the loop formed by the intermediate transfer belt 51 , opposite the driving roller 52 which is grounded. More specifically, the electric potential detector 58 faces a portion of the intermediate transfer belt 51 entrained around the driving roller 52 with a gap of approximately 4 mm. The surface potential of the toner image primarily transferred onto the intermediate transfer belt 51 is measured when the toner image comes to the position opposite the electric potential detector 58 . According to the present embodiment, a surface potential sensor EFS-22D manufactured by TDK Corp. is used as the electric potential detector 58 .
  • the fixing device 90 On the right side of the secondary transfer nip between the secondary transfer roller 53 and the intermediate transfer belt 51 , the fixing device 90 is disposed.
  • the fixing device 90 includes a fixing roller 91 and a pressing roller 92 .
  • the fixing roller 91 includes a heat source such as a halogen lamp inside thereof. While rotating, the pressing roller 92 pressingly contacts the fixing roller 91 , thereby forming a heated area called a fixing nip therebetween.
  • the recording medium P bearing an unfixed toner image on the surface thereof is delivered to the fixing device 90 and interposed between the fixing roller 91 and the pressing roller 92 in the fixing device 90 .
  • the toner adhered to the toner image is softened and affixed permanently to the recording medium P. Subsequently, the recording medium P is discharged outside the image forming apparatus from the fixing device 90 along a sheet passage after fixing process.
  • the secondary transfer bias power source 200 serving as a secondary transfer bias output device includes a direct current power source and an alternating current power source, and can output a superimposed bias as the secondary transfer bias.
  • the superimposed bias is composed of an alternating current voltage superimposed on a direct current voltage.
  • An output terminal of the secondary transfer bias power source 200 is connected to the metal cored bar of the secondary transfer roller 53 .
  • the level of the electric potential of the metal cored bar of the secondary transfer roller 53 is similar to or the same level as the output voltage of the secondary transfer bias power source 200 . Furthermore, the metal cored bar of the nip forming roller 56 is grounded. In this case, using toner having a negative charge polarity, a DC voltage having the same negative polarity as the toner is used so that the time-averaged potential of the superimposed bias has the same polarity as the toner, that is, the negative polarity. According to the present embodiment, an AC voltage has a zero-crossing waveform crossing 0V (zero volts).
  • the nip forming roller 56 is grounded while the superimposed bias is applied to the secondary transfer roller 53 .
  • the secondary transfer roller 53 may be grounded while the superimposed bias is applied to the nip forming roller 56 .
  • a DC voltage having the positive polarity which is an opposite polarity to the polarity of toner is used, and the time-averaged potential of the superimposed bias has the positive polarity that is a polarity opposite to the polarity of the toner.
  • the AC voltage has the zero-crossing waveform.
  • a DC voltage may be supplied to one of the secondary transfer roller 53 and the nip forming roller 56 while supplying an AC voltage to the other roller.
  • the DC voltage having the negative polarity same as the toner is used.
  • the DC voltage having the positive polarity which is a polarity opposite to the toner is used.
  • the AC voltage has the zero-crossing waveform.
  • toner having the positive charge polarity may also be used.
  • the polarity of the DC voltage is opposite to the polarity described above.
  • the alternating current voltage has the zero-crossing waveform.
  • an AC voltage having a sinusoidal waveform is used as the AC voltage.
  • an AC voltage having a rectangular waveform may be used.
  • FIG. 3 is a waveform chart showing an example of a waveform of a superimposed bias serving as the secondary bias output from the secondary transfer bias power source 200 .
  • a description is provided of a case in which the superimposed bias serving as a secondary transfer bias is supplied to the secondary transfer roller 53 .
  • a potential difference is treated as an absolute value.
  • the potential difference is treated as a value with polarity. More specifically, a value obtained by subtracting the potential of the metal cored bar of the nip forming roller 56 from the potential of the metal cored bar of the secondary transfer roller 53 is considered as the potential difference.
  • toner having the negative polarity as in the illustrative embodiment, when the polarity of the time-averaged value of the potential difference becomes negative, the potential of the nip forming roller 56 is increased beyond the potential of the secondary transfer roller 53 on the opposite polarity side to the polarity of charge on toner (the positive side in the present embodiment). Accordingly, the toner is electrostatically moved from the secondary transfer roller side to the nip forming roller side.
  • the left side of FIG. 3 illustrates separately the AC component and the DC component of the secondary transfer bias.
  • the AC component having a sinusoidal waveform is used and comprises a positive peak of +aV and a negative peak of ⁇ aV.
  • a peak-to-peak voltage Vpp of the AC component is 2aV.
  • the DC component of the voltage is ⁇ bV.
  • FIG. 3 illustrates the AC component and the DC component being superimposed.
  • an offset voltage Voff has the same level as the DC component of the superimposed bias.
  • the superimposed voltage consists of the AC component (Vpp) superimposed on the DC component (V), and the time-averaged value of the superimposed bias coincides with the offset voltage Voff.
  • the superimposed bias has thus a sinusoidal waveform being offset negatively and includes both the peak value “+(a ⁇ b)V” on the positive side and the peak value “ ⁇ (a+b)V” on the negative side.
  • the superimposed bias on the positive side above 0V acts on the toner such that the toner returns from the recording medium side to the belt side.
  • the superimposed bias on the negative side below 0V acts on the toner such that the toner moves from the belt side to the recording medium side in the secondary transfer nip.
  • Making the offset voltage Voff which is the time averaged value, the same polarity as the toner (here, negative polarity) enables the toner to move from the belt side to the recording medium comparatively, while moving back and forth reciprocally between the belt side and the recording medium side.
  • a recording medium having a coarse surface that is, having a high degree of surface roughness, such as an embossed sheet and a Japanese sheet
  • application of the superimposed bias enables the toner to move from the belt side to the recording medium comparatively while moving the toner reciprocally so that transferability of toner relative to the recessed portions on the recording medium is enhanced, hence preventing a disturbance of an image such as dropouts (blank spots).
  • the positive side of the superimposed bias contributes to enhancement of transferability relative to the coarse surface of the recording medium.
  • the negative side of the superimposed bias relates to what is needed for normal transfer (transfer of toner from the belt side to the recording medium side).
  • the image forming apparatus is capable of forming a full color or multiple-color image with at least toner of two colors in addition to a monochrome or single-color image using toner of a single color.
  • transfer of the full color or multiple-color image (hereinafter referred to simply as color image) containing a large amount of toner requires higher transferability.
  • FIG. 5 when carrying out the color mode for forming a color image consisting of a large amount of toner, levels of both the DC voltage and the AC voltage employed in the monochrome (single color) mode are changed such that a so-called return electric field that causes the toner to return from the recording medium side to the belt side is secured. More specifically, as illustrated in FIG. 5 , an area (crest) above 0V indicated by hatched lines on the positive side is secured (by an amount equal to or greater than the monochrome (single color) mode).
  • FIG. 5 is a waveform chart showing an example of the waveform of the superimposed bias in the color mode according to the illustrative embodiment of the present invention.
  • the transferability (the electric field to enable the toner to move from the belt side to the recording medium) is kept high by raising the offset voltage while maintaining the return electric field (the area above the 0V on the positive side) at the same level or greater than that in the single color mode. Accordingly, good transferability is achieved in the color mode at which a color image such as a full color image and a multiple color image that contains a large amount of toner is transferred onto the recording medium. Because the return electric field is secured, sufficient transferability relative to the recessed portions on the recording medium having a coarse surface can be achieved.
  • both the DC component and the AC component of the superimposed bias are voltage-controlled.
  • the DC component may be current-controlled.
  • a power source capable of current-control is generally expensive.
  • the AC component even when the DC component is current-controlled, the AC component is voltage-controlled.
  • Embodiment 1 A description is now provided of an example of the voltage control (Embodiment 1) and the current control (Embodiment 2).
  • Embodiment 2 both the DC component and the AC component are voltage-controlled.
  • Embodiment 2 the DC component is current-controlled.
  • the evaluation of the transferability was performed using a textured paper called “LEATHAC 66” (a trade name, manufactured by TOKUSHU PAPER MFG. CO., LTD.) having a ream weight of 130 Kg.
  • the transferability was graded on a five point scale of 1 to 5, where 5 is the highest grade in an organoleptic test. With the above-described voltages, the highest grade “5” was obtained.
  • the transferability in the color mode was graded using the same voltages, the transferability was graded as “1”, the lowest grade.
  • the toner can be moved reciprocally by securing the absolute value of the voltage on the toner return side at the same level or higher than that in the single color mode. With this configuration, good transferability is obtained.
  • Embodiment 2 a description is provided of Embodiment 2 in which the DC component is constant-current controlled (the AC component is constant-voltage controlled).
  • the current of the DC component of the superimposed bias in the monochrome mode (the single color mode) using the black toner was ⁇ 18 ⁇ A, and the peak-to-peak voltage of the AC component was 8 kV. Good transferability was obtained.
  • the transferability was graded on the five point scale of 1 to 5, where 5 is the highest grade in the organoleptic test. With the above-described current and voltage, the transferability was graded as “5”. However, when the transferability in the color mode was graded using the same condition, the transferability was graded as “1”, the lowest grade.
  • the transferability was graded as “5”. It is to be noted that the DC component was constant-current controlled, and the AC component was constant-voltage controlled. As compared with the constant-voltage controlled DC component, when the DC component is constant-current controlled, the ability to accommodate different environmental conditions and sheet types can be enhanced.
  • the image forming apparatus includes a controller 500 that controls the superimposed bias to be applied by the transfer bias power source.
  • the controller 500 changes, in the color mode, both levels of the DC component and the AC component of the superimposed bias from that in the single color mode such that the return electric field is secured in the superimposed bias in the color mode.
  • Embodiment 1 and Embodiment 2 are only an example using a test apparatus, and thus the levels of voltages and currents are not limited to the embodiments described above.
  • the voltages and currents may be set depending on the material of the components of the transfer device and the characteristics of toner.
  • the toner can be moved reciprocally by securing the same level of the return electric field or higher than that in the single color mode. Accordingly, good transferability is obtained. Further, good transferability is also obtained relative to the recording media sheets having a coarse surface.
  • the ability to accommodate different environmental conditions and different sheet types can be enhanced.
  • the bias is switched between the single color mode and the color mode.
  • the bias may be changed depending on different types of recording media sheets. For example, when using a recording medium having a coarse surface in the color mode, the voltage and/or the current of the superimposed bias greater than the values presented in the foregoing embodiments may be applied.
  • the type of the recording medium is selectable on a control panel of the image forming apparatus.
  • the type of the recording medium can be selected in the print setting of a host machine.
  • the color mode is also selectable.
  • the level of the voltage of the DC component is set to ⁇ 0.9 V and the peak-to-peak voltage of the AC component is set to 10 kV.
  • Embodiments 3 through 6 Similar to Embodiment 2, in Embodiments 3 through 6 the DC component is constant-current controlled while the AC component is constant-voltage controlled.
  • a test sheet A has a volume resistivity of 10.77 [Log ⁇ cm].
  • a surface resistivity of the front surface is 12.76 [Log ⁇ / ⁇ ].
  • the surface resistivity of the rear surface is 12.40 [Log ⁇ / ⁇ ].
  • the depth of a recessed portion of the surface is approximately 50 ⁇ m. The depth of the recessed portion refers to the longest distance between the highest peak and the lowest valley on the surface of the test sheet. The depth was measured using the laser microscope VK-9500 manufactured by Keyence Corporation.
  • the level of current of the DC component of the superimposed bias in the monochrome mode (single color mode) using the toner of black was ⁇ 40 ⁇ A, and the peak-to-peak voltage of the AC component was 3.7 kV.
  • the transferability was graded on the five point scale of 1 to 5, where 5 is the highest grade in the organoleptic test. With the above-described current and voltage, the transferability was graded as “5”.
  • the level of the voltage of the DC component when transferring the toner image onto the test sheet A was ⁇ 0.7 kV under a normal environment with the temperature of 23° C. and the relative humidity of 50%.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes. However, the fluctuation was within ⁇ 30% of ⁇ 0.7 kV.
  • the transferability in the color mode was evaluated using the same values, the transferability was graded as “1”.
  • the level of current of the DC component was increased to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component was increased to 6.2 kV.
  • the transferability was graded as “5”.
  • the level of the voltage of the DC component was ⁇ 1 kV under the normal environment with the temperature of 23° C. and the relative humidity of 50%.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes. However, the fluctuation was within ⁇ 30% of ⁇ 1 kV in a low-temperature, low-humidity environment as well as in a high-temperature, high-humidity environment.
  • the DC component was constant-current controlled while the AC component was constant-voltage controlled.
  • the DC component of the superimposed bias is constant-current controlled, the ability to accommodate different environmental conditions and different sheet types can be enhanced.
  • the image forming apparatus that controls transfer of a toner image using the superimposed bias
  • good transferability can be obtained in the color mode by changing both the DC component and the AC component of the superimposed bias of the single color mode to the color mode in which the toner image bears a large amount of toner so that the return electric field is secured in the superimposed bias.
  • the toner image can be transferred reliably onto the recessed portions of the recording medium having a high degree of surface roughness (coarse surface).
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes and so forth.
  • the absolute value (kV) of the voltage on the toner return side in the color mode was equal to or greater than that in the monochrome (single color) mode in a low-temperature, low-humidity environment as well as in a high-temperature, high-humidity environment.
  • a test sheet B has the volume resistivity of 10.96 [Log ⁇ cm].
  • the surface resistivity of the front surface is 13.10 [Log ⁇ / ⁇ ].
  • the surface resistivity of the rear surface is 13.25 [Log ⁇ / ⁇ ].
  • the depth of a recessed portion is approximately 100 ⁇ m.
  • the depth of the recessed portion refers to the longest distance between the highest peak and the lowest valley on the surface of the test sheet. The depth was measured using the laser microscope VK-9500 manufactured by Keyence Corporation.
  • the level of current of the DC component of the superimposed bias in the monochrome mode (single color mode) using the black toner was ⁇ 40 ⁇ A, and the peak-to-peak voltage of the AC component was 4.0 kV. Good transferability was obtained.
  • the transferability was graded on the five point scale of 1 to 5, where 5 is the highest grade in the organoleptic test. With the above-described current and voltage, the transferability was graded as “5”.
  • the level of the voltage of the DC component was ⁇ 0.7 kV under the normal environment with the temperature of 23° C. and the relative humidity of 50%.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes and so forth. However, the fluctuation was within ⁇ 30% of ⁇ 0.7 kV.
  • the transferability in the color mode was evaluated using the same values, the transferability was graded as “1”.
  • the level of the current of the DC component was raised to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component was raised to 6.4 kV, the transferability was graded as “5”.
  • the level of the voltage of the DC component was ⁇ 1.1 kV under the normal environment with the temperature of 23° C. and the relative humidity of 50%.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes and so forth. However, the fluctuation was within ⁇ 30% of ⁇ 1.1 kV in a low-temperature, low-humidity environment as well as a high-temperature, high humidity environment.
  • the DC component was constant-current controlled while the AC component was constant-voltage controlled.
  • the DC component of the superimposed bias is constant-current controlled, the ability to accommodate different environmental conditions and different sheet types can be enhanced.
  • the image forming apparatus that controls transfer of a toner image using the superimposed bias
  • good transferability can be obtained in the color mode by changing both the DC component and the AC component of the superimposed bias of the single color mode to the color mode in which the toner image bears a large amount of toner so that the return force of toner is secured in the superimposed bias.
  • the toner image can be transferred reliably onto the recessed portions of the recording medium having a high degree of surface roughness (coarse surface).
  • the transferability in the color mode was evaluated using the same voltages, the transferability was graded as “1”.
  • the toner can be moved reciprocally by securing the absolute value of the voltage on the toner return side at the same level or higher than that in the single color mode. With this configuration, good transferability is obtained.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes and so forth.
  • the absolute value (kV) of the voltage on the toner return side in the color mode was equal to or greater than that in the monochrome (single color) mode in a low-temperature, low-humidity environment as well as in a high-temperature, high-humidity environment.
  • a test sheet C has the volume resistivity of 11.18 [Log ⁇ cm].
  • the surface resistivity of the front surface is 12.99 [Log ⁇ / ⁇ ].
  • the surface resistivity of the rear surface is 13.11 [Log ⁇ / ⁇ ].
  • the depth of a recessed portion is approximately 80 ⁇ m.
  • the depth of the recessed portion refers to the longest distance between the highest peak and the lowest valley on the surface of the test sheet. The depth was measured using the laser microscope VK-9500 manufactured by Keyence Corporation.
  • the level of the current of the DC component of the superimposed bias in the monochrome mode (single color mode) using the black toner was ⁇ 40 ⁇ A, and the peak-to-peak voltage of the AC component was 4.3 kV.
  • the transferability was graded on the five point scale of 1 to 5, where 5 is the highest grade in the organoleptic test. With the above-described current and voltage, the transferability was graded as “5”.
  • the level of the voltage of the DC component was ⁇ 0.9 kV under the normal environment with the temperature of 23° C. and the relative humidity of 50%.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes and so forth. However, the fluctuation was within ⁇ 30% of ⁇ 0.9 kV.
  • the transferability in the color mode was evaluated using the same values, the transferability was graded as “1”.
  • the level of the current of the DC component was raised to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component was raised to 6.7 kV.
  • the transferability was graded as “5”.
  • the level of the voltage of the DC component was ⁇ 1.3 kV under the normal environment with the temperature of 23° C. and the relative humidity of 50%.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes and so forth. However, the fluctuation was within ⁇ 30% of ⁇ 1.3 kV in a low-temperature, low-humidity environment as well as a high-temperature and high-humidity environment.
  • the DC component was constant-current controlled while the AC component was constant-voltage controlled.
  • the DC component of the superimposed bias is constant-current controlled, the ability to accommodate different environmental conditions and different sheet types can be enhanced.
  • the image forming apparatus that transfers a toner image using the superimposed bias
  • good transferability can be obtained in the color mode by changing both the DC component and the AC component of the superimposed bias of the single color mode to the color mode in which the toner image bears a large amount of toner so that the return force of toner is secured in the superimposed bias.
  • the toner image can be transferred reliably onto the recessed portions of the recording medium having a high degree of surface roughness (coarse surface).
  • the toner can be move reciprocally by securing the absolute value of the voltage on the toner return side at the same level or higher than that in the monochrome or single color mode. With this configuration, good transferability is obtained.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes and so forth.
  • the absolute value (kV) of the voltage on the toner return side in the color mode was equal to or greater than that in the monochrome mode in a low-temperature, low-humidity environment as well as in a high-temperature, high-humidity environment.
  • a test sheet D has a volume resistivity of 10.92 [Log ⁇ cm].
  • the surface resistivity of the front surface is 12.62 [Log ⁇ / ⁇ ].
  • the surface resistivity of the rear surface is 12.37 [Log ⁇ / ⁇ ].
  • the depth of a recessed portion is approximately 110 ⁇ m.
  • the depth of the recessed portion refers to the longest distance between the highest peak and the lowest valley on the surface of the test sheet. The depth was measured using the laser microscope VK-9500 manufactured by Keyence Corporation.
  • the level of current of the DC component of the superimposed bias in the monochrome mode (single color mode) using the black toner was ⁇ 40 ⁇ A, and the peak-to-peak voltage of the AC component was 5.5 kV. Good transferability was obtained.
  • the transferability was graded on the five point scale of 1 to 5, where 5 is the highest grade in the organoleptic test. With the above-described current and voltage, the transferability was graded as “5”.
  • the level of the voltage of the DC component when transferring a toner image onto the test sheet D was ⁇ 1.4 kV under the normal environment with the temperature of 23° C. and the relative humidity of 50%.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes and so forth. However, the fluctuation was within ⁇ 30% of ⁇ 1.4 kV.
  • the transferability in the color mode was evaluated using the same values, the transferability was graded as “1”.
  • the transferability was graded as “5”.
  • the level of the voltage of the DC component when transferring a toner image onto the test sheet D was ⁇ 2.1 kV under the normal environment with the temperature of 23° C. and the relative humidity of 50%.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes and so forth. However, the fluctuation was within ⁇ 30% of ⁇ 2.1 kV in a low-temperature, low-humidity environment as well as high-temperature, high humidity environment.
  • the DC component was constant-current controlled while the AC component was constant-voltage controlled.
  • the DC component of the superimposed bias is constant-current controlled, the ability to accommodate different environmental conditions and different sheet types can be enhanced.
  • the image forming apparatus that transfers a toner image using the superimposed bias
  • good transferability can be obtained in the color mode by switching both the DC component and the AC component of the superimposed bias of the single color mode to the color mode in which the toner image bears a large amount of toner such that the return force of toner is secured in the superimposed bias.
  • the toner image can be transferred reliably onto the recessed portions of the recording medium having a high degree of surface roughness (coarse surface).
  • the transferability in the color mode was evaluated using the same values, the transferability was graded as “1”.
  • the toner can move reciprocally by securing the absolute value of the voltage on the toner return side at the same level or higher than that in the single color mode. Accordingly, good transferability is obtained.
  • the DC component of the bias was constant-current controlled so that the voltage at transfer fluctuated due to environmental changes and so forth.
  • the absolute value (kV) of the voltage on the toner return side in the color mode was equal to or greater than that in the monochrome mode in a low-temperature, low-humidity environment as well as in a high-temperature, high-humidity environment.
  • Embodiment 7 a description is provided of Embodiment 7 in which a plurality of modes for providing different superimposed biases corresponding to output images is provided.
  • the plurality of modes includes a normal mode, a halftone priority mode, and a solid image priority mode.
  • the halftone priority mode the peak-to-peak voltage of the AC component of the superimposed bias is less than the normal mode.
  • the solid image priority mode the peak-to-peak voltage of the AC component of the superimposed bias is greater than the normal mode.
  • An amount of toner per unit area (corresponding to a ratio of an image area of a recording medium) in an output image differs depending on images.
  • an optimum voltage and current by which the toner is transferred to the recording medium also change. More specifically, the optimum voltage and current refer to the voltage and current by which the toner is transferred well comparatively to the recessed portions of the recording medium while moving the toner reciprocally, thereby preventing degradation of transferability and an image defect such as dropouts.
  • a user or a technician chooses, in accordance with an image to be output, a proper print mode from the plurality of print modes on the control panel of the image forming apparatus or a print setting of a host machine.
  • a proper print mode from the plurality of print modes on the control panel of the image forming apparatus or a print setting of a host machine.
  • the halftone priority mode is selected in a case of an image having a low toner density.
  • the solid image priority mode is selected.
  • the superimposed bias in the monochrome (single color) mode with the color black is set as ⁇ 40 ⁇ A for the current of the DC component and 3.7 kV for the peak-to-peak voltage of the AC component.
  • the current of the DC component is set to ⁇ 40 ⁇ A and the peak-to-peak voltage of the AC component is set to 3.2 kV.
  • the current of the DC component is set to ⁇ 40 ⁇ A and the peak-to-peak voltage of the AC component is set to 4.6 kV.
  • the superimposed bias is set to ⁇ 70 ⁇ A for the current of the DC component, and 6.2 kV for the peak-to-peak voltage of the AC component.
  • the current of the DC component is set to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component is set to 5.4 kV.
  • the current of the DC component is set to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component is set to 7.0 kV.
  • the superimposed bias in the monochrome (single color) mode with the color black is set to ⁇ 40 ⁇ A for the current of the DC component, and 4.0 kV for the peak-to-peak voltage of the AC component.
  • the current of the DC component is set to ⁇ 40 ⁇ A and the peak-to-peak voltage of the AC component is set to 3.3 kV.
  • the solid image priority mode is selected, the current of the DC component is set to ⁇ 40 ⁇ A and the peak-to-peak voltage of the AC component is set to 4.9 kV.
  • the superimposed bias is set to ⁇ 70 ⁇ A for the current of the DC component, and 6.4 kV for the peak-to-peak voltage of the AC component.
  • the current of the DC component is set to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component is set to 5.6 kV.
  • the current of the DC component is set to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component is set to 7.3 kV.
  • the superimposed bias in the monochrome (single color) mode with the color black is set to ⁇ 40 ⁇ A for the current of the DC component and 4.3 kV for the peak-to-peak voltage of the AC component.
  • the current of the DC component is set to ⁇ 40 to and the peak-to-peak voltage of the AC component is set to 3.6 kV.
  • the current of the DC component is set to ⁇ 40 ⁇ A and the peak-to-peak voltage of the AC component is set to 5.5 kV.
  • the superimposed bias when the normal mode is selected is set as ⁇ 70 ⁇ A for the current of the DC component, and 6.7 kV for the peak-to-peak voltage of the AC component.
  • the current of the DC component is set to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component is set to 6.0 kV.
  • the current of the DC component is set to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component is set to 8.0 kV.
  • the superimposed bias in the monochrome (single color) mode with the color black is set to ⁇ 40 ⁇ A for the current of the DC component, and 5.5 kV for the peak-to-peak voltage of the AC component.
  • the current of the DC component is set to ⁇ 40 ⁇ A and the peak-to-peak voltage of the AC component is set to 4.1 kV.
  • the current of the DC component is set to ⁇ 40 ⁇ A and the peak-to-peak voltage of the AC component is set to 6.5 kV.
  • the superimposed bias is set to ⁇ 70 ⁇ A to for the current of the DC component, and 8.9 kV for the peak-to-peak voltage of the AC component.
  • the current of the DC component is set to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component is set to 7.9 kV.
  • the current of the DC component is set to ⁇ 70 ⁇ A and the peak-to-peak voltage of the AC component is set to 10.0 kV.
  • toner can be transferred well with an optimum voltage and current corresponding to the amount of toner by adjusting the superimposed transfer bias in accordance with an output image.
  • the toner is transferred well to the recessed portions of the recording medium, thereby preventing degradation of transferability and an image defect such as dropouts.
  • the secondary transfer nip is formed by interposing the secondary transfer belt 51 between the secondary transfer roller 53 and the nip forming roller 56 contacting pressingly against the secondary transfer roller 53 .
  • a belt-type nip forming member (conveyance belt, also known as a transfer belt) may be employed.
  • the secondary transfer portion may employ a contact-free system. More specifically, a contact-free transfer charger serving as a transfer device is disposed facing the secondary transfer roller 53 without contacting the secondary transfer roller 53 .
  • the polarity of the DC component of the superimposed bias is the opposite polarity to the polarity of the charge on toner.
  • the toner image on the intermediate transfer belt 51 is transferred onto the recording medium delivered between the secondary transfer roller 53 and the intermediate transfer belt 51 , and the transfer charger by absorbing the toner image to the recording medium.
  • the image forming apparatus employs an intermediate transfer method in which the toner image formed on the photosensitive member is transferred primarily onto the intermediate transfer belt, and then transferred onto a recording medium.
  • the image forming apparatus may employ a direct transfer method in which the toner image formed on the photosensitive member is transferred directly onto a recording medium as illustrated in FIG. 6 .
  • FIG. 6 is a cross-sectional diagram schematically illustrating an image forming apparatus of the direct transfer method.
  • the recording medium is fed onto a conveyance belt 131 by a sheet feed roller 32 , and the toner images on photosensitive drums 2 Y, 2 C, 2 M, and 2 K are transferred directly onto the recording medium by transfer rollers 25 Y, 25 C, 25 M, and 25 K, respectively, such that they are superimposed one atop the other, thereby forming a composite toner image.
  • the composite toner image is fixed by the fixing device 90 .
  • the conveyance belt 131 is formed into a loop and entrained about support rollers 132 and 133 .
  • the transfer rollers 25 K, 25 M, 25 C, and 25 Y are connected to power sources 81 K, 81 M, 81 C, and 81 Y, respectively.
  • a superimposed bias in which an AC voltage is superimposed on a DC voltage is used as the transfer bias to be applied to each of the transfer portions.
  • both the DC component and the AC component of the superimposed bias of the single color mode are changed to the color mode such that the return electric field is secured in the superimposed bias.
  • FIG. 7 is a cross-sectional diagram schematically illustrating a color image forming apparatus using a single photosensitive member.
  • the image forming apparatus of this type includes one photosensitive drum 201 surrounded by a charging device 203 , a primary transfer roller 205 , and developing devices 204 Y, 204 M, 204 C, and 204 K for the colors yellow, magenta, cyan, and black, respectively.
  • the surface of the photosensitive drum 201 is charged uniformly by the charging device 203 . Subsequently, the charged surface of the photosensitive drum 201 is illuminated with a light beam L modulated based on image data associated with the color yellow. Accordingly, an electrostatic latent image for the color yellow is formed on the surface of the photosensitive drum 201 .
  • the developing unit 204 Y develops the electrostatic latent image for yellow with yellow toner, thereby forming a toner image of yellow.
  • the toner image of yellow formed on the photosensitive drum 201 is transferred primarily onto an intermediate transfer belt 206 by the primary transfer roller 205 .
  • residual toner remaining on the photosensitive drum 201 is cleaned by a drum cleaner 220 .
  • the surface of the photosensitive drum 201 is uniformly charged by the charging device 203 in preparation for the subsequent imaging process.
  • the surface of the photosensitive drum 201 is illuminated with a light beam L modulated based on image data associated with the color magenta. Accordingly, an electrostatic latent image for the color magenta is formed on the surface of the photosensitive drum 201 .
  • the developing unit 204 M develops the electrostatic latent image for magenta with magenta toner, thereby forming a toner image of magenta.
  • the toner image of magenta formed on the photosensitive drum 206 is transferred onto the intermediate transfer belt 206 , such that the toner image of magenta is superimposed on the toner image of yellow.
  • the toner images of cyan and black are transferred primarily onto the intermediate transfer belt 206 in the similar manner as the color magenta, thereby forming a composite toner image.
  • the recording medium is conveyed to a secondary transfer nip at which the intermediate transfer belt 206 is interposed between a secondary transfer roller 209 and a nip forming roller 207 , and the composite color toner image is transferred onto the recording medium.
  • the recording medium bearing the composite toner image thereon is delivered to a fixing device 400 .
  • the surface of the intermediate transfer belt 206 is cleaned by a belt cleaner 222 in preparation for subsequent imaging process.
  • heat and pressure are applied to the recording medium to fix the composite toner image on the recording medium. After fixing, the recording medium is output onto a sheet discharge tray, not illustrated.
  • the secondary transfer portion of the single-drum type color image forming apparatus is constituted by the secondary transfer roller 209 and the nip forming roller 207 . Similar to the image forming apparatus shown in FIG. 1 , the nip forming roller 207 is grounded while the superimposed bias is supplied to secondary transfer roller 209 . As described above, in the color mode, both the DC component and the AC component of the superimposed bias of the single color mode are changed to the color mode such that the return electric field is secured in the superimposed bias.
  • FIG. 8 is a schematic diagram illustrating a variation of the transfer portion. The same effect as that of the foregoing embodiments can be achieved with this configuration.
  • toner images formed on photosensitive drums 701 are primarily transferred onto a belt-type intermediate transfer member 702 (hereinafter referred to simply as an intermediate transfer belt).
  • the intermediate transfer belt 702 contacts a secondary transfer conveyance belt 703 , thereby forming a transfer nip.
  • the toner image is transferred onto a recording medium P at the transfer nip.
  • the recording medium P After the recording medium P is fed by a pair of registration rollers 706 , the recording medium P passes through the transfer nip between the intermediate transfer belt 702 and the secondary transfer conveyance belt 703 . As the recording medium P passes through the transfer nip, the toner image is transferred onto the recording medium P, and then the recording medium P separated from the intermediate transfer belt 702 is delivered to a fixing device (not illustrated) by the secondary transfer conveyance belt 703 .
  • a first roller 704 disposed inside the loop formed by the intermediate transfer belt 702 may serve as a bias application roller to which a bias having the opposite polarity to the charge (normal charging polarity) on toner is applied. This is known as a repulsive force transfer method.
  • a second roller 705 disposed inside the loop formed by the secondary transfer conveyance belt 703 opposite the first roller 704 may serves as a bias application roller to which a bias having the same polarity as the toner (normal charging polarity) is applied. This is known as an attraction transfer method.
  • a transfer bias roller and/or a bias application brush may be disposed inside the loop formed by the secondary transfer conveyance belt 703 , and the transfer bias is applied to the transfer bias roller and/or the bias application brush.
  • the transfer bias roller and/or the bias application brush may be disposed below the transfer nip or near the transfer nip but downstream from the transfer nip.
  • the transfer roller may include a foam layer (elastic layer) or a surface layer coated with elastic material such as foam.
  • the transfer charger may be employed.
  • the configuration of the transfer portion is not limited to the configuration described above.
  • the second roller side may be substituted by a belt member.
  • the contact-free method using the charger may be employed. Any suitable power source such as a known power source may be employed as a power source for outputting the superimposed bias.
  • the configuration of the image forming apparatus is not limited to the configuration described above.
  • the order of image forming units arranged in tandem is not limited to the above-described order.
  • the present invention may be applied to an image forming apparatus using toners in three different colors or less.
  • the present invention is employed in the image forming apparatus.
  • the image forming apparatus includes, but is not limited to, an electrophotographic image forming apparatus, a copier, a printer, a facsimile machine, and a multi-functional system.
  • any one of the above-described and other exemplary features of the present invention may be embodied in the form of an apparatus, method, or system.
  • any of the aforementioned methods may be embodied in the form of a system or device, including, but not limited to, any of the structure for performing the methodology illustrated in the drawings.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Electrostatic Charge, Transfer And Separation In Electrography (AREA)
  • Color Electrophotography (AREA)
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US20130004190A1 (en) 2013-01-03

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